performance: Refactor how data is managed to reduce copying. * Make use of `collections.abc.Mapping` to better identify things that are supposed to look like dictionaries. * Instead of handling "overlay" of data in a dict tree in each different data object, make all objects `Mapping`s and handle merging at a higher level with the new `MergedMapping` object. * Since this new object is read-only, remove the need for deep-copying of app and page configurations. * Split data classes into separate modules.

--- a/piecrust/rendering.py	Sat Jun 27 22:28:32 2015 -0700
+++ b/piecrust/rendering.py	Sun Jun 28 08:22:39 2015 -0700
@@ -282,7 +282,7 @@
         data_ctx.pagination_filter = ctx.pagination_filter
         page_data = build_page_data(data_ctx)
         if ctx.custom_data:
-            page_data.update(ctx.custom_data)
+            page_data._appendMapping(ctx.custom_data)
         return page_data